Discovered by MacRumors, Apple’s description of the feature is the following:
Reading List lets you collect webpages and links for you to read later. To add the current page to your Reading List, click Add Page. You can also Shift-click a link to quickly add it to the list. To hide and show Reading List, click the Reading List icon (eyeglasses) in the bookmarks bar.
The feature is currently inactive within the latest build of Lion, so those running the release won’t be able to try it out just yet, but as a massive Instapaper addict I can’t wait for Reading List to go live.